I. Field of the Invention
The invention relates to a carbon electrode, in particular a graphite electrode for producing steel, comprising a metal-containing coating to increase the electrical conductivity and/or to decrease the oxidation loss on the lateral electrode surface.
Metal-containing protective coatings have been developed in order to reduce surface oxidation of graphite electrodes and, hence, to reduce the consumption of the surface of the electrode. These protective coatings consist for example of carbides, silicides, oxides, metals or combinations of such materials and are applied onto the electrode surface by spraying and by subsequently being burned-in by means of an electric arc, whereby they are securely connected to the electrode surface.
A coating consisting of aluminium and silicon carbide has proven to be particularly useful for this purpose. After the coating has been applied, the graphite electrode has a metallic, rough coat, the conductivity of which nearly corresponds to that of the aluminum, and which securely adheres to the graphite surface because of the preceeding electric arc treatment. Starting at about 500.degree. C., the protective coating becomes slightly plastic, so that it then becomes impermeable to gas. With increasing temperature, it is gradually converted to oxide beneath the electrode holder, but it retains its protective function.
The advantage attained by such coated graphite electrodes, namely the reduction consumption and erosion of the surface of the electrode is opposed by a considerable drawback resulting from the fact that the working material of the electrode holder must be matched to the requirements of the metallic coating surface introduction of the electrode through the furnace cover must be such that the coating is not scraped off. Therefore, in order to avoid fusion of the contact jaws (customarily consisting of copper) with the metallic coating of the graphite electrodes, the copper contacts must be replaced by appropriate contacts, such as those made of graphite. Furthermore, it has been necessary to employ a well effective sealing ring for each electode, so that more extensive contaminations of the coating and of the contacts are avoided.
These types of re-constructions of furnaces needed when changing over to coated electrodes have a considerable cost.
Now even though it has become known from German OS No. 22 51 590 to spray graphite a as heat-resistant material onto an electrode surface coated with aluminum in a process for producing protective coatings on objects made of carbon, this measure nevertheless only serves to increase the melting point of the coating. After the graphite has been applied, this higher-melting coating is formed by fusion of the graphite to the aluminum using an electric arc.
A similar use of graphite is known in connection with a process for producing aluminium-containing protective coatings on carbon products (German AS No. 1 646 679), wherein a spread of graphite is applied onto the aluminum previously sprayed onto the carbon article. This graphitic spread is then directly exposed to processing in the electric-arc furnace, whereupon the surface of the thus produced coating is formed.